Generally, CRT monitors operate by focusing and accelerating thermal electrons emitted from red (R), green (G) and blue (B) electron guns, colliding the thermal electrons to R, G and B phosphor layers through each point on a shadow mask to form a pixel, and forming a two-dimensional image by applying sawtooth waveform current to vertical and horizontal deflection coils.
CRT monitors form images by repeatedly scanning electron beams in horizontal and vertical directions. Phosphor dots provided on a screen emit light when electron beams passing through holes of a shadow mask collide therewith.
When the dots emit light, a ripple pattern or fringe pattern can appear on the monitor screen due to interference between a shadow mask and a scan pattern, which is referred to as the “moire phenomenon”. The moire phenomenon occurs due to the interference between wavelengths, and moire increases as the resolution and clarity of an image increases.
Various video modes can be input into a CRT monitor and therefore, the CRT monitor is expected to appropriately display according to the various video modes. For example, horizontal synchronous signals having various frequencies in a range from 30 KHz to 90 KHz can be input to a personal computer (PC). In this situation, if a horizontal synchronous signal having a predetermined moire wavelength is input to a CRT monitor, moire having a ripple pattern appears on the CRT monitor in a vertical direction.
Some of the moire can be removed by changing the resolution or frequency of an image or the period of a horizontal synchronous signal. It is difficult to completely remove the moire, however, because the moire phenomenon will vary between different monitors (even if the monitors are the same type of CRT monitor), and the amount of moire can vary between the upper and lower portions of a single monitor.
Conventional methods for removing moire use the same control value throughout a screen to change the period of a horizontal synchronous signal and consequently, moire may partially remain on the screen, and the remaining moire cannot be completely removed.